[GHC] #12098: Typechecker regression in 8.0.1

[GHC]

#12098: Typechecker regression in 8.0.1
-------------------------------------+-------------------------------------
           Reporter:  vagarenko      |             Owner:
               Type:  bug            |            Status:  new
           Priority:  highest        |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
  (Type checker)                     |
           Keywords:                 |  Operating System:  Windows
       Architecture:  x86            |   Type of failure:  GHC rejects
                                     |  valid program
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 This code:

 {{{#!hs
 {-# LANGUAGE IncoherentInstances #-}
 {-# LANGUAGE RankNTypes          #-}
 {-# LANGUAGE RoleAnnotations     #-}

 module Bug where

 import Data.Proxy (Proxy(..))
 import GHC.Prim   (coerce)

 class Throws e where {}
 type role Throws representational

 newtype Wrap e a = Wrap { unWrap :: Throws e => a }

 coerceWrap :: Wrap e a -> Wrap (Catch e) a
 coerceWrap = coerce

 newtype Catch a = Catch a
 instance Throws (Catch e) where {}

 unthrow :: Proxy e -> (Throws e => a) -> a
 unthrow _ = unWrap . coerceWrap . Wrap
 }}}

 compiles fine with ghc 7.10.2 but fails with ghc 8.0.1 with error:

 {{{
 Bug.hs:25:13: error:
     * Could not deduce (Throws e)
       from the context: Throws e0
         bound by a type expected by the context:
                    Throws e0 => a
         at Bug.hs:25:13-38
       Possible fix:
         add (Throws e) to the context of
           the type signature for:
             unthrow :: Proxy e -> (Throws e => a) -> a
     * In the expression: unWrap . coerceWrap . Wrap
       In an equation for `unthrow':
           unthrow _ = unWrap . coerceWrap . Wrap
 }}}

 This code is extracted from blog post http://www.well-
 typed.com/blog/2015/07/checked-exceptions/ and gist
 https://gist.github.com/edsko/f1f566f77422398fba7d

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12098>
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